Chemical snapshot unveils path to greener biofuel

Vehicle fuels made of plant waste are sustainable and climate friendly. Unfortunately the energy in stems, bark and twigs is locked up in cellulose, which is tough to crack open by the enzymes used to transform cellulose into sugar, which can then be fermented into alcohol.

One family of enzymes, lytic polysaccharide monooxygenases (LPMOs), ease the transformation of cellulose. They are the way forward. Chemists at the University of Copenhagen have now taken a leap ahead in understanding how LPMOs work by showing how these enzymes bind to cellulose. This can be incredibly important for, among other things, the development and production of sustainable biofuels.

Kristian Frandsen is a PhD student at the University of Copenhagen's Department of Chemistry. Together with Associate Professor Lo Leggio and Laboratory Manager Jens-Christian Navarro Poulsen, he is part of CESBIC, an international research consortium that includes the Danish company Novozymes and others. Today, the research team will have their article 'The molecular basis of polysaccharide cleavage by lytic polysaccharide monooxygenases'published in the prestigious journal Nature Chemical Biology.

In large part, plant waste consists of cellulose, an energy rich polymer made up of sugar (glucose), bound together in chains. Glucose is fermented into alcohol that can be used as fuel. Kristian Frandsen, the article's first author, explains that LPMO eases the path to cellulose for other enzymes, thus making it easier for them to ultimately break the cellulose down. Indeed, understanding the mechanics of this process is crucial.

"We are the first ones to get a picture of an LPMO in the first stage of the breakdown process, and in high resolution no less. Combined with our colleagues' biochemical and spectroscopic insights, the entire team of researchers has been able to attain a detailed appreciation of the chemical mechanisms. That is, how the enzyme is able to hack away at cellulose at the sub-atomic level." Frandsen hopes that these insights will make it easier to optimise production of new and even more effective enzymes. "Additionally, the project has provided me with contact to many international experts," says Frandsen.

Frandsen's PhD supervisor, Associate Professor Lo Leggio, refers to the work as a breakthrough. While the first LPMOs were identified at the end of the 1990's, they were first suspected to be glycoside hydrolases, a type of enzyme that breaks bonds using water molecules. It was later discovered that these enzymes used oxygen (known as oxidation). At the same time, it was also discovered that a copper ion was essential to the process.

"We had known what the enzymes looked like on their own for quite some time. This knowledge was important as form controls function. But many in the LPMO field consider the understanding of how LPMOs bind to cellulose as some sort of "Holy Grail". Without that knowledge, it is impossible to understand the details of what controls the reaction. With X-ray crystallography, we have now been able to take a few snapshots in crystals of LPMOs that were soaked with bits of cellulose, allowing us to witness detail at the atomic level," explains Lo Leggio.

The UCPH researchers have used X-ray crystallography to shed light on the LPMO enzymes' interaction with cellulose. In order to do this, the cellulose fragments needed to be bound into crystals of the enzyme. Jens-Christian Navarro Poulsen, the group member responsible for the crystallization laboratory, explains:

"Our group had already conducted unsuccessful attempts with other LPMOs to create a crystal in which the substrate, cellulose, was bound. And we know that many other groups have conducted similar experiments. Therefore, it was a great moment when we received the good news from Kristian that cellulose fragments were apparent in the structure," says Poulsen.

The vital X-ray experiments were conducted at the MAXlab in Lund, Sweden and at the ESRF facility in Grenoble, France.

In addition to UCPH's Department of Chemistry and Novozymes, the company that identified and produced the enzyme for the experiments, the partners of the international consortium are the University of Cambridge, Aix-Marseille Université and the University of York.

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